Modified chondroitin synthase polypeptide and crystal thereof

ABSTRACT

Disclosed are: (A) a polypeptide consisting of the amino acid sequence of SEQ ID NO:2, or (B) a polypeptide comprising an amino acid sequence of SEQ ID NO:2 including deletion, substitution or addition of one or several amino acid residues and having chondroitin synthase activity; a nucleic acid encoding the polypeptide; a method for producing the polypeptide, comprising at least the steps of: (1) expressing the nucleic acid to produce the polypeptide; and (2) collecting the polypeptide produced in the step (1); and a crystal of the polypeptide. The crystal may be a monoclinic or tetragonal crystal.

TECHNICAL FIELD

The present invention relates to a modified chondroitin synthasepolypeptide, a nucleic acid encoding the polypeptide, a method ofproducing the polypeptide, a crystal of the polypeptide, and the like.

BACKGROUND ART

Abbreviations, to be used herein and meanings thereof are as follows.

K4CP: Chondroitin synthase derived from Escherichia coli K4 strain(serotype O5:K4(L):H4, ATCC 23502)

GalNAc: N-acetyl-D-galactosamine

GlcUA: D-glucuronic acidSDS: Sodium dodecyl sulfateSDS-PAGE: Sodium dodecyl sulfate-polyacrylamide gel electrophoresisUDP: Uridine 5′-diphosphate

Patent Document 1 discloses a chondroitin sulfate synthase derived fromEscherichia coli K4 strain and a DNA encoding the synthase. PatentDocument 2 discloses a modified enzyme having a substitution of one orseveral amino acids in a certain region of K4CP (SEQ ID NO:4).

However, both of the documents do not disclose and suggest thepolypeptide of the present invention, nucleic acid encoding thepolypeptide, crystal of the polypeptide of the present invention, andthe like. In addition, any one of the documents does not disclose andsuggest ideas of modifying K4CP (SEQ ID NO:4) to increase the expressionlevel from its DNA, to enhance the enzymatic activity, and to facilitatecrystallization.

Patent Document 3 discloses a novel chondroitin sulfate lyase derivedfrom Proteus vulgaris and a crystal thereof. However, there is nodisclosure or suggestion on K4CP (SEQ ID NO:4).

-   Patent Document 1: JP 2003-199583 A-   Patent Document 2: JP 2005-65565 A-   Patent Document 3: JP 10-262660 A

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide: a chondroitin synthasepolypeptide which can be expressed at a high level from a nucleic acid,has a high enzymatic activity, and can be crystallized; a nucleic acidencoding the polypeptide; a method of producing the polypeptide; acrystal of the polypeptide; and the like.

The inventors of the present invention have made extensive studies, andas a result, they have obtained a polypeptide with a deletion in aspecific region of K4CP (SEQ ID NO:4) and a nucleic acid encoding thepolypeptide. The inventors have surprisingly found that use of thepolypeptide as a chondroitin synthase can significantly increaseefficiency of the expression from its nucleic acid and can significantlyenhance the enzymatic activity and that the peptide can be easilycrystallized because the polypeptide molecule is stable, therebycompleted the present invention.

That is, the present invention provides a polypeptide represented by thefollowing (A) or (B) (hereinafter, referred to as “polypeptide of thepresent invention”):

(A) a polypeptide consisting of the amino acid sequence of SEQ ID NO: 2;or

(B) a polypeptide consisting of an amino acid sequence of SEQ ID NO: 2including deletion, substitution, or addition of one or several aminoacids and having chondroitin synthase activity.

In addition, the present invention provides a nucleic acid encoding thepolypeptide represented by the following (A) or (B) (hereinafter,referred to as “nucleic acid of the present invention”):

(A) a polypeptide consisting of the amino acid sequence of SEQ ID NO: 2;or

(B) a polypeptide consisting of an amino acid sequence of SEQ ID NO: 2including deletion, substitution, or addition of one or several aminoacids and having chondroitin synthase activity.

As the nucleic acid of the present invention, a nucleic acid asdescribed in the following (a) or (b) may be exemplified:

(a) a DNA consisting of the nucleotide sequence of SEQ ID NO: 1; or

(b) a DNA that hybridizes with a DNA consisting of the nucleotidesequence complementary to the DNA (a) under stringent conditions and haschondroitin synthase activity.

In addition, the present invention provides a method of producing thepolypeptide of the present invention, comprising at least the followingsteps (1) and (2) (hereinafter, referred to as “production method of thepresent invention”):

(1) a step of expressing a polypeptide from at least one of the nucleicacids of the present invention; and

(2) a step of collecting the polypeptide expressed in the step (1).

In addition, the present invention provides a crystal of the polypeptideof the present invention (hereinafter, referred to as “crystal of thepresent invention”).

As the crystal of the present invention, a plate crystal (hereinafter,referred to as “crystal 1 of the present invention”) may be exemplified.Specifically, a crystal showing the following crystal data may beexemplified:

Crystal system: monoclinic system

Bravais lattice: primitive monoclinic lattice

Space group: P2₁

Lattice constant:

a=83.5 Å

b=232.0 Å

c=86.0 Å

β=105.5°

In addition, as the crystal of the present invention, an octahedralcrystal (hereinafter, referred to as “crystal 2 of the presentinvention”) may also be exemplified. More specifically, a crystalshowing the following crystal data may be exemplified:

Crystal system: tetragonal system

Bravais lattice: primitive tetragonal lattice

Space group: P4

Lattice constant:

a=336 Å

b=336 Å

c=100 Å

In addition, the present invention provides a method of producingchondroitin comprising reacting a sugar receptor substrate, aD-glucuronic acid donor, and an N-acetyl-D-galactosamine donor in thepresence of the polypeptide of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing (picture) showing an SDS-PAGE of K4CP (SEQ ID NO:4)treated with trypsin.

FIG. 2 is a drawing (picture) showing an SDS-PAGE of the fractions ofgel filtration chromatography of ΔN57K4CP (SEQ ID NO:2).

FIG. 3 is a drawing (picture) showing an optical microscope image ofcrystal 1 of the present invention.

FIG. 4 is a drawing (picture) showing an optical microscope image ofcrystal 2 of the present invention.

FIG. 5 is a graph showing elution patterns of chondroitins synthesizedby using ΔN57K4CP (SEQ ID NO:2).

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention is described in detail by referringto the best mode for carrying out the invention.

<1> Polypeptide of the Present Invention

The polypeptide of the present invention is a polypeptide represented bythe following (A) or (B):

(A) a polypeptide consisting of the amino acid sequence of SEQ ID NO: 2;or

(B) a polypeptide consisting of an amino acid sequence of SEQ ID NO: 2including deletion, substitution, or addition of one or several aminoacids and having chondroitin synthase activity.

Hereinafter, explanations are made in order.

Polypeptide (A)

The polypeptide (A) is a K4CP (SEQ ID NO:4) polypeptide in which 57amino acid residues from the N-terminal are deleted, and the amino acidsequence of the polypeptide is shown in SEQ ID NO: 2. The method ofproducing the polypeptide is not particularly limited as long as apolypeptide having the sequence can be obtained. For example, thepolypeptide can be produced by a peptide synthesis technique, or byremoving 57 amino acid residues from the N-terminal with a protease suchas trypsin, or by a genetic engineering technique using a nucleic acidencoding the polypeptide. More specific examples are shown in Examples 1and 2 below.

The polypeptide has a high chondroitin synthase activity and can becrystallized easily as described below.

Polypeptide (B)

The polypeptide (B) is a polypeptide consisting of an amino acidsequence of SEQ ID NO: 2 including deletion, substitution, or additionof one or several amino acids and having chondroitin synthase activity.

In a polypeptide, polymorphism or mutation in a nucleic acid encodingthe polypeptide may occur, and mutation which causes deletion,substitution, or addition of amino acids may also occur in its aminoacid sequence due to the modification of the produced polypeptide incells and in purification. Despite this, it is known that a certainpeptide shows a physiological/biological activity that is substantiallyequal to that of a polypeptide having no mutation. The polypeptide (B)includes a polypeptide having a structure slightly different from thepolypeptide (A) but having no significant difference in the function.

Examples of the addition include addition of one or two or more aminoacid residues selected from methionine, alanine, and glycine to theamino terminal of SEQ ID NO: 2 and addition of a peptide sequence forpurification or a spacer sequence as described below to the aminoterminal or carboxy terminal of SEQ ID NO: 2.

Note that the “amino acid sequence of SEQ ID NO: 2 including addition ofone or several amino acids” in (B) does not include the sequence withaddition of 57 amino acid residues from the N-terminal of K4CP (SEQ IDNO:4) or a part thereof at the amino terminal of SEQ ID NO: 2, whichcauses a loss of an expression efficiency and crystallization propertiesspecific to the polypeptide of the present invention.

The term “several amino acids” as used herein refers to the number ofamino acid residues that may be mutated as long as the chondroitinsynthase activity is not impaired. Specifically, the number is, forexample, an integer of 2 to 40, preferably an integer of 2 to 30, morepreferably an integer of 2 to 20, still more preferably an integer of 2to 15, still more preferably an integer of 2 to 10, still morepreferably an integer of 2 to 8, and still more preferably an integer of2 to 6.

The polypeptide of the present invention may have an amino acid sequencehaving not less than 90%, preferably not less than 95%, more preferablynot less than 98% homology to SEQ ID NO: 2 as long as the peptide dosenot include 57 amino acid residues from the N-terminal of K4CP (SEQ IDNO:4), and the peptide has chondroitin synthase activity. The homologyof amino acid sequences may be determined based on an algorithm byKarlin and Altschul, BLAST (Proc. Natl. Acad. Sci. USA, 90, 5873 (1993))or FASTA (Methods Enzymol., 183, 63 (1990)).

The polypeptide of the present invention has chondroitin synthaseactivity. Whether the peptide has the chondroitin synthase activity ornot can be verified in accordance with the method described in Example 3below.

The method of producing the polypeptide is not particularly limited, andthe peptide can be produced by a peptide synthesis technique based on anamino acid sequence of SEQ ID NO: 2 including deletion, substitution, oraddition of one or several amino acids (having chondroitin synthaseactivity) or by a genetic engineering technique using a nucleic acidhaving a mutation introduced therein by the method described below.

The polypeptide of the present invention can be used for synthesis ofchondroitin (elongation reaction of chondroitin sugar chain) bycontacting the polypeptide with sugar donor substrates (UDP-GalAc andUDP-GluUA) and a sugar receptor substrate (chondroitin oligosaccharide)for the synthesis. In addition, the polypeptide of the present inventioncan be used as a material for producing the crystal of the presentinvention to be described below. Details are shown in Examples below.

<2> Nucleic Acid of the Present Invention

The nucleic acid of the present invention is a nucleic acid encoding thefollowing polypeptide (A) or (B):

(A) a polypeptide consisting of the amino acid sequence of SEQ ID NO: 2;or

(B) a polypeptide consisting of an amino acid sequence of SEQ ID NO: 2including deletion, substitution, or addition of one or several aminoacids and having chondroitin synthase activity.

The polypeptides (A) and (B) are as described in “<1> Polypeptide of thepresent invention” above.

The term “nucleic acid” as used herein includes DNA and RNA. Therefore,the kinds of the nucleic acid of the present invention may be DNA or RNAas long as the nucleic acid encodes the aforementioned polypeptide, andin particular, the nucleic acid is preferably a DNA.

A person skilled in the art would easily understand that the nucleicacid of the present invention includes nucleic acids having variousnucleotide sequences because of degeneracy of genetic codes.

As the nucleic acid of the present invention, a nucleic acid asdescribed in the following (a) or (b) may be exemplified:

(a) a DNA consisting of the nucleotide sequence of SEQ ID NO: 1; or

(b) a DNA that hybridizes with a DNA having the nucleotide sequencecomplementary to the DNA (a) under stringent conditions and haschondroitin synthase activity.

The term “stringent conditions” as used herein refers to conditionswhere so-called specific hybrid is formed and non-specific hybrid is notformed (see Sambrook, J. et al., Molecular Cloning A Laboratory Manual,second Edition, Cold Spring Harbor Laboratory Press (1989), etc.).Specific examples of the “stringent conditions” include conditions forhybridization at 42° C. in a solution containing 50% formamide, 4×SSC,50 mM HEPES (pH 7.0), 10×Denhardt's solution, 100 μg/ml salmon sperm DNAand washing with 2×SSC, 0.1% SDS solution at room temperature andwashing with 0.1×SSC, 0.1% SDS solution at 50° C. Whether thepolypeptide has chondroitin synthase activity or not can be verified inaccordance with the method described in Example 3 below.

The nucleic acid of the present invention has been obtained based on thenucleic acid originally isolated from Escherichia coli K4 strain butincludes nucleic acids produced by nucleic acid synthesis, geneticengineering technique or the like. As described above, although themethod of producing the nucleic acid of the present invention is notparticularly limited, the nucleic acid may be produced by the methoddescribed in Examples below.

Among the nucleic acids of the present invention, a nucleic acidencoding the polypeptide (B) and the DNA (b) may be produced as follows.

Into the nucleic acid encoding the polypeptide (A) or the DNA (b),deletion, substitution, or addition of nucleotides to cause deletion,substitution, or addition of amino acid residues, which does notsubstantially impair the chondroitin synthase activity of thepolypeptide encoded by such nucleic acids, is introduced. The deletion,substitution, or addition of nucleotides may be introduced into thenucleic acid by synthesizing a sequence which has restrictionenzyme-cleavable sites at both ends and contains both side portions ofthe mutated position, and replacing the corresponding nucleotidesequence contained in the non-mutated nucleic acid with the synthesizedsequence. Alternatively, the deletion, substitution, or addition can bealso introduced into the nucleic acid in accordance with a method suchas site-specific mutagenesis method (Kramer, W. and Frits, H. J., Meth.in Enzymol., 154, 350 (1987); Kunkel, T. A. et al., Meth. in Enzymol.,154, 367 (1987)). If a polypeptide, expressed from a nucleic acid havingdeletion, substitution, or addition introduced as described above, haschondroitin synthase activity, the nucleic acid can be confirmed to bethe nucleic acid of interest. Whether the polypeptide expressed haschondroitin synthase activity or not can be verified in accordance withthe method described in Example 3 below.

If the nucleic acid of the present invention, which encodes thepolypeptide of the present invention, is expressed, the polypeptide ofthe present invention can be produced. Details are shown in Examplesbelow.

<3> Production Method of the Present Invention

The production method of the present invention is a method of producingthe polypeptide of the present invention including at least thefollowing steps (1) and (2):

(1) a step of expressing the polypeptide from at least one of thenucleic acids of the present invention; and

(2) a step of collecting the polypeptide expressed in the step (1).

Hereinafter, explanations are made in order.

The step (1) is a step of expressing a polypeptide from the nucleic acidof the present invention. The nucleic acid of the present invention isas described in “<2> Nucleic acid of the present invention” above.

The method of expressing a polypeptide from the nucleic acid of thepresent invention is not particularly limited as long as the polypeptideencoded by the nucleic acid of the present invention is produced fromthe nucleic acid. For example, a polypeptide can be expressed from thenucleic acid of the present invention by: inserting the nucleic acid ofthe present invention into an appropriate expression vector; introducingthe vector into an appropriate host to prepare a transformant, andgrowing the transformant. The expression vector and host are notparticularly limited and may be appropriately selected from knownvectors and hosts by a person skilled in the art. Specific examplesthereof are shown in Examples below. The term “growing” as used hereinincludes not only proliferation of cells or microorganisms serving astransformants but also growing of animals, insects; and the like whichhave been incorporated with the cells serving as transformants. Thegrowth conditions may be appropriately selected by a person skilled inthe art depending on the type of a host to be used.

The polypeptide expressed in this step is the polypeptide of the presentinvention.

The step (2) is a step of collecting the polypeptide (the polypeptide ofthe present invention) expressed in the step (1).

The method of collecting the polypeptide can be appropriately selectedfrom known methods by a person skilled in the art depending on theexpression method of the polypeptide in the step (1).

For example, when the polypeptide of the present invention is expressedand secreted in a medium (the supernatant of the culture broth) byinserting the nucleic acid of the present invention into an expressionvector, introducing the vector into a host such as Escherichia coli, andculturing the host, the medium may be harvested and used without furthertreatment as the polypeptide of the present invention.

In addition, when the polypeptide is expressed as a soluble formsecreted in the cytoplasm or as an insoluble (membrane-bound) form, thepolypeptide expressed may be extracted by a treatment procedure such as:a method using a nitrogen cavitation apparatus; extraction by cell lysissuch as homogenization, a glass bead mill method, an ultrasonicdisruption method, and an osmotic pressure shock method, or afreezing-thawing method; surfactant extraction; or a combinationthereof. Alternatively, the extract may be harvested and used as thepolypeptide of the present invention without further treatment.

The production method of the present invention may further includeanother step as long as the method includes the steps (1) and (2).

For example, a step of purifying the polypeptide of the presentinvention may be performed after the step (2). Purification may bepartial purification or complete purification, and the method may beappropriately selected by a person skilled in the art depending on thepurpose of the polypeptide of the present invention or the like.

Specific examples of the purification method include treatmentprocedures such as salting-out with ammonium sulfate, sodium sulfate, orthe like, centrifugation, dialysis, ultrafiltration, adsorptionchromatography, ion-exchange chromatography, hydrophobic chromatography,reverse-phase chromatography, gel filtration, gel permeationchromatography; affinity chromatography, electrophoresis, andcombination thereof.

The polypeptide of the present invention may be expressed as a fusionprotein with other proteins or peptides such asglutathione-S-transferase (GST) or polyhistidine to purify the peptideusing an affinity column for the proteins or peptides. Such fusionprotein is also included in the polypeptide of the present invention.

Whether the polypeptide of the present invention is produced or not canbe confirmed by analyzing the amino acid sequence, functions, and thelike of the resultant polypeptide.

<4> Crystal of the Present Invention

The crystal of the present invention is a crystal of the polypeptide ofthe present invention. That is, the crystal includes the crystals ofboth the polypeptides (A) and (B) above. The crystal is preferably thecrystal of the polypeptide (A).

The crystal of the present invention is not particularly limited as longas the crystal is a product of crystallization of the polypeptide of thepresent invention. In addition, the method of producing the crystal maybe appropriately selected from known techniques used for crystallizationof polypeptides by a person skilled in the art. Examples of the crystalof the present invention include monoclinic and tetragonal crystals.

Specific examples of the crystal of the present invention includecrystals of the present invention 1 and 2.

The crystal 1 of the present invention is preferably a plate crystal. Acrystal showing the following crystal data is more preferred:

Crystal system: monoclinic system

Bravais lattice: primitive monoclinic lattice

Space group: P2₁

Lattice constant:

a=83.5 Å

b=232.0 Å

c=86.0 Å

β=105.5°

The crystal 1 of the present invention can be produced by addingpolyethyleneglycol to a solution containing the polypeptide of thepresent invention at a final concentration of about 15% and allowing thesolution to stand at room temperature. More specific production methodis as shown in Example 4 below.

The crystal 2 of the present invention is preferably an octahedralcrystal. A crystal showing the following crystal data is more preferred:

Crystal system: tetragonal system.

Bravais lattice: primitive tetragonal lattice

Space group: P4

Lattice constant:

a=336 Å

b=336 Å

c=100 Å

The crystal 2 of the present invention can be produced by adding sodiumformate to a solution containing the polypeptide of the presentinvention at a final concentration of about 4 M and allowing thesolution to stand at room temperature. More specific production methodis as shown in Example 5 below.

The crystal of the present invention can provide the polypeptide of thepresent invention at very high purity. For example, crystal of thepresent invention may be redissolved in an aqueous solvent and used asthe polypeptide of the present invention for chondroitin synthesis orthe like.

<5> Method of Producing Chondroitin of the Present Invention

The method of producing chondroitin of the present invention includes astep of reacting a sugar receptor substrate, a D-glucuronic acid donor(GlcUA), and an N-acetyl-D-galactosamine donor(GalNAc) in the presenceof the polypeptide of the present invention.

The GlcUA donor is preferably nucleoside diphosphate-GlcUA, andparticularly preferably UDP-GlcUA. On the other hand, the GalNAc donoris preferably nucleoside diphosphate-GalNAc, particularly preferablyUDP-GalNAc.

The sugar receptor substrates are preferably chondroitin, andparticularly preferably chondroitin oligosaccharides such as chondroitintetrasaccharide and chondroitin hexasaccharide. A purified chondroitincan be obtained by producing chondroitin by the above-described reactionand purifying the resultant chondroitin by column chromatography, forexample.

The method for producing chondroitin of the present invention is notlimited by the molecular weight of the resultant chondroitin, andchondroitin having a molecular weight as high as hundreds of thousandsor more can be produced as described in Examples below.

Hereinafter, the present invention is described in detail by referringto the examples.

Example 1

Production of polypeptide of the present invention (ΔN57K4CP) (SEQ IDNO:2) by trypsin treatment of wild-type K4CP (SEQ ID NO:4).

A DNA having the nucleotide sequence of SEQ ID NO: 3 (encoding wild-typeK4CP (SEQ ID NO:4)) was inserted between a BamHI site (nucleotidenumbers 930 to 935 in SEQ ID. NO: 5) and an EcoRI site (nucleotidenumbers 938 to 943 in SEQ ID NO: 5) in pGEX4T3 (manufactured by AmershamBiosciences; the nucleotide sequence was shown in SEQ ID NO: 5) as anexpression vector. The DNA having the nucleotide sequence of SEQ ID NO:3 was prepared by the method described in Patent Document 1 (“insert” inExample 2 of Patent Document 1 corresponds to the DNA having thenucleotide sequence of SEQ ID NO: 3).

The DNA was used to transform Escherichia coli Top10F′ (Invitrogen), andthe polypeptide was expressed in the same way as the method described inExample 2 of Patent Document 1.

The polypeptide expressed by pGEX4T3 was obtained as a fusion proteinwith GST (glutathione S-transferase), and the polypeptide expressed wassubjected to affinity purification using Glutathione Sepharose 4B (GEHealthcare Biosciences).

The purified polypeptide was treated with thrombin to remove GST. Thetreatment with thrombin was performed by adding 1 unit of thrombin to 13μg of the polypeptide and incubating the mixture at 4° C. overnight.

After the thrombin treatment, gel filtration (Sephacryl-S200;manufactured by Amersham Biosciences) was performed, to thereby obtainK4CP containing no GST (having the amino acid sequence of SEQ ID NO: 4).

The resultant product was treated with trypsin at a final concentrationof 10 ng/ml at 20° C. for 15 minutes. Untreated and treated solutionswere subjected to SDS-PAGE (12.5% gel), and the gel was stained withCoomassie brilliant blue. The results are shown in FIG. 1. In FIG. 1,“M”, “−”, and “+” represent a molecular weight marker, atrypsin-untreated solution, and a trypsin-treated solution,respectively.

As shown in FIG. 1, in the case of the trypsin-treated solution, a bandhaving a slightly smaller molecular weight produced by trypsin treatment(the solid triangle mark in FIG. 1) was detected. As a result of ananalysis of an N-terminal amino acid sequence of the band having thesmaller molecular weight, the band was found to correspond to a K4CP(SEQ ID NO:4) molecule in which 57 amino acid residues from theN-terminal has been deleted. Therefore, the band was found to be thepolypeptide having the amino acid sequence of SEQ ID NO: 2. Hereinafter,the polypeptide is referred to as “ΔN57K4CP” (SEQ ID NO:2).

Example 2

Production of the polypeptide of the present invention (ΔN57K4CP) (SEQID NO:2) by genetic engineering technique

A DNA having the nucleotide sequence of SEQ ID NO: 1 (encoding ΔN57K4CP)(SEQ ID NO:2) was inserted between a BamHI site (nucleotide numbers 930to 935 in SEQ ID NO: 5) and an EcoRI site (nucleotide numbers 938 to 943in SEQ ID′NO: 5) in the above-mentioned expression vector, pGEX4T3. TheDNA having the nucleotide sequence of SEQ ID NO: 1 was prepared by PCR(polymerase chain reaction).

The DNA was introduced into Escherichia coli Top10F′ (Invitrogen), andthe bacterium was cultured at 30° C. in 2×YT medium supplemented withampicillin sodium at a final concentration of 100 μg/ml. When theconcentration of the bacterial cells in the culture medium (OD600;absorbance at 600 nm) reached 0.6 to 1, IPTG(isopropyl-β-D(−)-thiogalactopyranoside) was added to the culture mediumat a final concentration of 0.1 mM, followed by culture at 30° C.overnight. After completion of culture, a supernatant of a cellhomogenate was subjected to affinity chromatography using GlutathioneSepharose 4B (GE Healthcare Biosciences).

The purified polypeptide was treated with thrombin to remove GST. Thetreatment with thrombin was performed in the same way as in Example 1.

After the thrombin treatment, gel filtration was performed in the sameway as in Example 1, to thereby obtain ΔN57K4CP (SEQ ID NO:2) containingno GST (having the amino acid sequence of SEQ ID NO: 2). The elutedfractions (7 fractions) obtained by the gel filtration were subjected toSDS-PAGE in the same way as in Example 1, and the gel was stained withCoomassie brilliant blue. The results are shown in FIG. 2. In FIG. 2,“M” represents a molecular weight marker.

FIG. 2 shows that ΔN57K4CP (SEQ ID NO:2) can be produced by the geneticengineering technique. An expression level of ΔN57K4CP (SEQ ID NO:2)expressed in this example was found to be about ten times higher than anexpression level of K4CP (SEQ ID NO:4) expressed in Example 1.

Example 3

Measurement of Enzymatic Activity

The chondroitin synthase activities of both ΔN57K4CP (SEQ ID NO:2)produced in Example 2 and K4CP (SEQ ID NO:4) produced by the methoddescribed in Example 2 of Patent Document 1 (obtained by fusing apeptide containing a His-tag with an N-terminal domain of thepolypeptide having the amino acid sequence of SEQ ID NO: 4) weremeasured. The measurement method is as follows.

A reaction solution (50 μl) containing the polypeptide (2.0 μg) to bemeasured for its enzymatic activity, 50 mM Tris-HCl (pH 7.2), 20 mMMnCl₂, 0.15 M NaCl, chondroitin hexasaccharide (0.1 nmol; SeikagakuCorporation), UDP-[³H]GalNAc (0.1 μCi, 3 nmol), and UDP-GlcUA (3 nmol)was incubated at 30° C. for 30 minutes. After incubation, the reactionsolution was heated at 100° C. for one minute to terminate the enzymaticreaction. After that, the solution was applied to Superdex PeptideHR10/30 column (GE Healthcare Biosciences), and gel filtrationchromatography was performed using 0.2 M NaCl as an eluent. Fractionscontaining chondroitin hexasaccharide or larger molecules werecollected, and the radioactivity of [³H]GalNAc incorporated by theenzymatic reaction was measured using a scintillation counter.

It was found that the chondroitin synthase activity of ΔN57K4CP (SEQ IDNO:2) was 2.06 times larger than that of K4CP (SEQ ID NO:4).

Example 4 Production of Crystal 1 of the Present Invention

A solution containing ΔN57K4CP (SEQ ID NO:2) obtained in Example 2 at aconcentration of 20 mg/ml (solvent composition: 50 mM Tris-HCl (pH 8.0),500 mM NaCl, 10 mM UDP, and 5 mM MnCl₂) was prepared, andpolyethyleneglycol (PEG3350; Hampton Research) and NaCl were added tothe solution at final concentrations of 15% and 0.2 M, respectively, andthe whole was allowed to stand at room temperature. As a result, acrystal of ΔN57K4CP (SEQ ID NO:2) was produced. An optical microscopeimage of the crystal is shown in FIG. 3. As shown in FIG. 3, the crystalis a plate crystal. X-ray crystallographic analysis was performed forthe crystal to obtain the following crystal data.

Crystal system: monoclinic system

Bravais lattice: primitive monoclinic lattice

Space group: P2₁

Lattice constant:

a=83.5 Å

b=232.0 Å

c=86.0 Å

β=105.5°

Example 5 Production of Crystal 2 of the Present Invention

A solution containing ΔN57K4CP (SEQ ID NO:2) obtained in Example 2 at aconcentration of 20 mg/ml (solvent composition: 50 mM Tris-HCl (pH 8.0),50 mM NaCl, 10 mM UDP, and 5 mM MnCl₂) was prepared, and sodium formateand dithiothreitol were added to the solution at final concentrations of4 M and 20 mM, respectively, and the whole was allowed to stand at roomtemperature. As a result, a crystal of ΔN57K4CP (SEQ ID NO:2) wasproduced. An optical microscope image of the crystal is shown in FIG. 4.As shown in FIG. 4, the crystal is an octahedral crystal. X-raycrystallographic analysis was performed for the crystal to obtain thefollowing crystal data.

Crystal system: tetragonal system

Bravais lattice: primitive tetragonal lattice

Space group: P4

Lattice constant:

a=336 Å

b=336 Å

c=100 Å

Although crystallization of K4CP of SEQ ID NO: 4 (where 57 amino acidresidues were not removed from the N-terminal) were tried by variousmethods, but K4CP (SEQ ID NO:4) crystal was not obtained.

Example 6

ΔN57K4CP (SEQ ID NO: 2) (20 μg) produced in Example 2 was incubated inreaction solutions (50 mM Tris-HCl, pH 7.2, 0.2 mM MnCl₂, 0.15 M NaCl,100 μl) containing UDP-[³H]GalNAc (0.1 μCi, 30 nmol), UDP-GlcUA (30nmol), and chondroitin hexasaccharide ([A] 1 nmol, [B] 0.1 nmol, [C]0.01 nmol) at 30° C. for 72 hours. The reaction solutions were heatedand subjected to gel filtration chromatography at a flow rate of 0.5ml/min using Sephacryl 5500 HR10/30 column and Superose 6 HR10/30 column(GE Healthcare Biosciences), which were connected in series. [³H]radioactivities of eluted fractions were measured to analyze elutionpatterns of the synthesized chondroitins (FIG. 5).

A calibration curve of molecular weights was created using a hyaluronicacid reference standard, and average molecular weights of thesynthesized chondroitins eluted were calculated based on the calibrationcurve. As a result, the average molecular weights of [A], [B], and [C]were found to be about 16 kDa, about 216 kDa, and about 685 kDa,respectively.

Example 7

ΔN57K4CP (SEQ ID NO:2) (200 μg) produced in Example 2 was incubated inthe reaction solutions (100 μl) as used in Example 6 containingUDP-GalNAc having no radioactivity (300 nmol), UDP-GlcUA (300 nmol), andchondroitin hexasaccharide ([A] 10 nmol, [B] 1 nmol, [C] 0.1 nmol) toperform enzymatic reaction. The reaction solutions were heated andapplied to Fast Desalting column (GE Healthcare Biosciences), and elutedfractions of higher molecular weights were passed through C18 Sep-Pak(Waters), followed by freeze-drying. Average molecular weights of theresultant synthesized chondroitins in Table 1 shown below, referred toas “rCH”) were measured using a multi-angle laser light scatteringdetector (Wyatt Technology Corporation), and as a result, the molecularweights of [A], [B], and [C] were found to be 34.7 kDa, 157 kDa, and 344kDa, respectively (Table 1).

TABLE 1 Measurement of molecular weight of rCH by light scatteringdetector First Second Sample name D/A measurement measurement Average[A] 30 31.8k  37.6k  34.7k  [B] 300 156k 158k 157k [C] 3,000 380k 308k344k

In this table, “D/A” represents a molar concentration ratio of asugar-nucleotide donor substrate with respect to chondroitinhexasaccharide. The reaction was performed at 30° C. for 72 hours.

It was found from the results of Examples 6 and 7 that chondroitinhaving a higher molecular weight can be produced efficiently by usingΔN57K4CP (SEQ ID NO:2).

INDUSTRIAL APPLICABILITY

The polypeptide of the present invention can be used as a tool or thelike for producing chondroitin efficiently and inexpensively. It is veryuseful because the polypeptide can be expressed from a nucleic acid veryefficiently compared with wild-type K4CP (SEQ ID NO:4), and it has anenzymatic activity higher than that of wild-type K4CP (SEQ ID NO:4) andcan be easily crystallized. Moreover, chondroitin having a highermolecular weight can be produced efficiently by using the polypeptide ofthe present invention. The nucleic acid of the present invention is veryuseful because the nucleic acid can be used as a tool for producing thepolypeptide of the present invention efficiently and inexpensively. Theproduction method of the present invention is very useful because themethod can be used for efficient production of the polypeptide of thepresent invention. The crystal of the present invention is very usefulbecause the crystal can provide the polypeptide of the preset inventionat extremely high purity.

1.-10. (canceled)
 11. An isolated or purified nucleic acid that encodesa polypeptide that is at least 90% homologous to the amino acid sequenceof SEQ ID NO:
 2. 12. The isolated or purified nucleic acid of claim 11that encodes a polypeptide that is at least 95% homologous to the aminoacid sequence of SEQ ID NO:
 2. 13. The isolated or purified nucleic acidof claim 11 that encodes a polypeptide that is at least 98% homologousto the amino acid sequence of SEQ ID NO:
 2. 14. The isolated or purifiednucleic acid of claim 11 that encodes a polypeptide consisting of theamino acid sequence of SEQ ID NO: 2 except that it includes deletion,substitution, or addition of one or two amino acids.
 15. The isolated orpurified nucleic acid of claim 11 that encodes a polypeptide comprisingthe amino acid sequence of SEQ ID NO:
 2. 16. The isolated or purifiednucleic acid of claim 11 that encodes a polypeptide that has chondroitinsynthase activity.
 17. The isolated or purified nucleic acid of claim 11that comprises the nucleotide sequence of SEQ ID NO:
 1. 18. The isolatedor purified nucleic acid of claim 11 that hybridizes with a DNAconsisting of the nucleotide sequence complementary to thepolynucleotide sequence of SEQ ID NO: 1 under stringent conditions whichcomprise washing with 0.1×SSC and 0.1% SDS at a temperature of 50° C.19. An isolated or purified vector comprising the purified nucleic acidof claim
 11. 20. A host cell containing the vector of claim
 19. 21. Amethod for producing a polypeptide having chondroitin synthase activitycomprising: expressing the nucleic acid sequence of claim 11 to form apolypeptide and recovering the expressed polypeptide.
 22. The method ofclaim 21, wherein said nucleic acid sequence is expressed in a hostcell.